The present invention relates to a novel method for the preparation of polyurethane elastomers by use of an aromatic diamine compound as the curing agent. In particular, the invention relates to a method for the preparation of polyurethane elastomers by use of a polyethyleneglycol-bis(4-aminobenzoate) as the curing agent.
It is well known that polyurethane elastomers are usually prepared by the reaction of a polyisocyanate and a polyol admixed with a curing agent or by the reaction of a curing agent with a liquid polyurethane prepolymer having isocyanate groups at the chain ends prepared in advance by the reaction of a polyol and an excessive amount of a polyisocyanate.
As a consequence of expanded application fields of polyurethane polymers in recent years, there is a growing demand for large quantities of polyurethane-based materials to be used for making large-sized shaped articles of polyurethane elastomers, for water-proofing in civil engineering and building construction, as pavement materials with elasticity, as sealing materials and the like. When a polyurethane composition is to be used in these applications, it is an essential requirement that the ready-prepared composition obtained by the admixture of the curing agent have a sufficiently long pot life in order to facilitate smooth casting or coating with the composition to give products or coating of uniform quality over a given work time. Some of the examples of the conventional curing agents are aromatic diamine compounds such as 4,4'-methylene-bis(2-chloroaniline), 3,3'-dichlorobenzidine and the like, of which the first mentioned compound is widely employed since excellent polyurethane elastomers with a high hardness and elasticity as well as good abrasion resistance are obtained with this curing agent.
The problem with 4,4'-methylene-bis(2-chloroaniline) is that the polyurethane composition when admixed with this curing agent has a rather short pot life leading to insufficient workability for fabrication of large articles or mass use in civil engineering or building construction.
In order to overcome this defect of 4,4'-methylene-bis(2-chloroaniline), several kinds of improved curing agents are proposed in Japanese Patent Disclosure SHO No. 50-132096 including alkylenediol-bis(4-aminobenzoates) and the like.
The problem with the alkylenediol-bis(4-aminobenzoates), on the other hand, is that the melting points of these compounds are so high that it is necessary to melt the curing agent beforehand in order to mix it with the prepolymer which should also be preheated at an elevated temperature in order to avoid precipitation of the curing agent during mixing. This necessity for heating of the curing agent and/or the prepolymer is disadvantageous not only due to the cost of heating and lower workability but also due to the danger of frequent degradation of the prepolymer by the prolonged heating at an elevated temperature resulting in inferior qualities of the final products.
Thus there have hitherto been known no curing agents for polyurethane elastomers satisfying all of the above described requirements with respect to the melting point, pot life of the ready-prepared compositions and the properties of the final polyurethane products.